Kinetics of the uninhibited and ethanol-inhibited CoO,Co2O3 and Ni2O3 catalyzed autoxidation of sulfur(IV) in alkaline medium

For getting an insight into the mechanism of atmospheric autoxidation of sulfur(IV), the kinetics of this autoxidation reaction catalyzed by CoO, Co₂O₃ and Ni₂O₃ in buffered alkaline medium has been studied, and found to be defined by Eqs. I and II for catalysis by cobalt oxides and Ni₂O₃, respectively.

(I)

(II)

The values of empirical rate parameters were: A{0.22(CoO), 0.8 L mol⁻¹s⁻¹ (Co₂O₃)}, K ₁{2.5 × 10² (Ni₂O₃)}, K ₂{2.5 × 10²(CoO), 0.6 × 10² (Co₂O₃)} and k ₁{5.0 × 10⁻²(Ni₂O₃), 1.0 × 10⁻⁶(CoO), 1.7 × 10⁻⁵ s⁻¹(Co₂O₃)} at pH 8.20 (CoO and Co₂O₃) and pH 7.05 (Ni₂O₃) and 30 °C. This is perhaps the first study in which the detailed kinetics in the presence of ethanol, a well known free radical scavenger for oxysulfur radicals, has been carried out, and the rate laws for catalysis by cobalt oxides and Ni₂O₃ in the presence of ethanol were Eqs. III and IV, respectively.

(III)

(IV)

For comparison, the effect of ethanol on these catalytic reactions was studied in acidic medium also. In addition, alkaline medium, the values of the inhibition factor C were 1.9 × 10⁴ and 4.0 × 10³ L mol⁻¹ s for CoO and Co₂O₃, respectively; for Ni₂O₃, C was only 3.0 × 10² only. On the other hand, in acidic medium, the values of this factor were all low: 20 (CoO), 0.7 (Co₂O₃) and 1.4 (Ni₂O₃). Based on these results, a radical mechanism for CoO and Co₂O₃ catalysis in alkaline medium, and a nonradical mechanism for Ni₂O₃ in both alkaline and acidic media and for cobalt oxides in acidic media are proposed.